The breastfed human infant ingests more than 10 degrees macrophages each day. The biology and function of the human milk macrophage will be evaluated using short- and long-term tissue culture of fresh and cryopreserved samples to determine the role these cells may have in maternal and neonatal host defense and immunologic development. The kinetics of lysosyme production in these cells will be determined by radioimmunoassay. The ability of the cells to influence lymphocyte response to lectins, B cell proliferation and antibody production will be evaluated using assays of thymidine incorporation, fluorescence microscopy and radioimmunoassay. Response of macrophages from milk to lymphokines will be tested by evaluating their basal and stimulated state of phagocytic, metabolic and bactericidal activity using sensitized erythrocytes and metabolic assays. An explanation for the lack of linear motility of these cells will be sought with ultrastructural studies, fluorescence microscopy, ionophore and calcium and magnesium flux studies. Preliminary findings that milk supernatant factors accelerate differentiation of the human blood monocyte to a macrophage will be further explored both by attempting to characterize the active factor(s) and by determining if these morphologic observations have functional or biochemical correlates.